Low cost elevator systems, such as those found in low rise apartment houses, warehouses and garages, are characterized by very simple, basic car motion control: basically start and stop. Unlike more sophisticated systems, where the car velocity is regulated to obtain a short "flight time" between floors without discomfort to the passengers, less costlier systems do not provide any form of car velocity control. Instead, car velocity is uncontrollably determined by the load: as the load increases, the car ascends slower and descends faster. One slight variation from this occurs in two speed A.C. systems: In those systems motor speed is switched from a high speed to a slowdown speed as the car is stopped. But this does not actually control the car velocity to any real extent because in either mode the load still alters the car velocity.
A typical car stopping sequence in a single speed system is this: As the elevator car approaches the floor, the motor is deactivated, at some distance from the floor level, and the brake is applied; the car then slides to a stop; ideally it smoothly stops precisely at the floor level. The operation of a two speed system is slightly different in that there the motor is slowed at a first distance from the floor level and then, at a second distance, which is closer to the floor, the motor is stopped and the brake is applied. But regardless which of the systems is used, if the car speed varies, that type of performance is very difficult to obtain because in most systems the stopping operation begins at a predetermined distance from the floor level, and that distance is usually determined by considering the maximum car velocity which occurs under full load down and no load up conditions. The maximum car velocity is obviously used because that dictates the maximum stopping distance required to position the car precisely at the floor. But this imposes a prescribed, unalterable car position at which the stopping action commences, regardless of the actual car velocity and, consequently, if the car velocity is less than the predetermined maximum velocity, the car will not stop precisely at the floor level, simply because less stopping distance is required at that lower velocity. If this happens, the car may undershoot the floor and the car then has to be moved slightly. The uneven ride often associated with low cost systems stems from such performance.
Owing to this limitation, a number of remedial approaches have been taken. These approaches, however, have added considerably to cost and complexity of the systems and have not proven to be extremely accurate or reliable, especially for two speed systems. They may be characterized as those which mechanically respond to motor speed and those which electrically measure changes in power consumption; to detect changes in car velocity.
Generally speaking, the mechanical arrangements operate like a governor. As motor speed increases, it causes a contact, which energizes the brake, to be variably positioned, in proportion to the speed, from a cam which is moved to actuate the contact, in order to energize the brake. Depending on the motor speed, the distance required for the cam to move varies in proportion to the motor speed (hence, the load) and thus brake operation is delayed also in proportion to the load. This type of system, however, is not inexpensive, principally due to its rather complex structure. Furthermore, it also imposes a significant maintenance expense for the same reasons.
Those systems following the electrical approach measure the power consumed by the motor; the power varies, of course, roughly in proportion to the load. In its most simple form a power measuring system merely diverts a portion of the motor current through a transformer which produces a voltage at a transformer output. The magnitude of that voltage varies according to the motor current and therefore the load. The magnitude of the transformer output signal is utilized in conventional ways to variably control a timing circuit that initiates the stopping operation. Among the disadvantages of this scheme is that it has very poor sensitivity to actual load variations. Typically, the speed range for an A.C. motor used in a low cost elevator system is between 1000 and 1500 R.P.M.; yet the change in speed between maximum and minimum load conditions is frequently no more than 80 R.P.M. That requires a speed variation sensitivity of roughly 8%. A system of the power measuring type having that sensitivity is expensive. A further complication in the application of these systems is that the speed regulation of the motors in low cost systems is typically coarse (no better than 5% to 7%). Consequently, changes in motor speed due to poor motor speed regulation frequently are mistakenly sensed by these systems as changes in load. Another significant limitation with these systems is their susceptibility to erroneous readings due to high starting currents and line surges and, because of that, it is often necessary to sense both the motor current and its phase angle. But this adds to the complexity of the system and therefore its cost. For all of these reasons, this type of load compensating system is not a very attractive approach in inexpensive elevator systems.
Because these prior art approaches raise reliability, accuracy and service concerns, and add considerable cost, it is not surprising that they have not found wide acceptance as a solution to the car positioning problems found in low cost elevator systems, especially those using single and two speed A.C. motors. Thus there still is a need for a simple, highly reliable, low cost system for controlling the car stopping functions in relation to the car load, and manifestly a system which is equally suited for both retrofit and new equipment applications. Those needs are fulfilled by the method and apparatus of the present invention.